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chapter_5_stereochemistry

chapter_5_stereochemistry - Chapter 5 Stereochemistry The...

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Chapter 5 Stereochemistry
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head2down The study of the 3-dimensional structure of molecules head2down Stereoisomers: have the same bonding sequence, but differ in the orientation of their atoms in space 191
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Chirality and Enantiomers (5-2) head2down Chiral objects : are those that have right-handed and left-handed forms The chirality of an object can be determined by looking at its mirror image 192
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head2down A chiral object has a mirror image that is different from the original object (it is non-superimposable) Figure 5-1 193 Chiral Achiral : object that is not chiral
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Molecules can either be chiral or achiral Figure 5-6 Chiral compound Figure 5-7 Achiral compound 194
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In B : the original molecule and its mirror image are non- A B Figure 5-3 195 superimposable ( no plane of symmetry ) Chiral In A : the original molecule and its mirror image are superimposable ( plane of symmetry ) Achiral
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head2down Two molecules are said to be superimposable if they can be placed on top of each other and the 3-D position of each atom on one molecule coincides with the equivalent atom on the other molecule head2down Molecules that are non-superimposable 196 mirror images are called: Enantiomers
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Nomenclature of Asymmetric Carbon Atoms (5-3) head2down For a carbon atom to be chiral, it must have 4 different substituents . In this case the carbon atom is called: Figure 5-6 197 Chiral carbon Chiral centre Asymmetric carbon Stereocentre
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head2down Asymmetric carbons are marked with a * Figure 5-4 198
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head2down Generalizations If a compound has no chiral carbon , it is usually achiral If a compound has just one chiral carbon , it is always chiral 199 If a compound has more than one chiral carbon , it may or may not be chiral
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Rule of thumb: Any molecule that has an internal plane of symmetry cannot be chiral, even though it may contain chiral carbon atoms Figure 5-3 200
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head2down Because enantiomers are actually 2 distinct molecules with different properties, a notation (nomenclature) system for naming configurations of chiral carbon atoms was proposed by: Cahn-Ingold-Prelog 201 It assigns a letter ( R ) or ( S ) to the chiral carbon
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Procedure to assign “R” and “S” Assign a priority to each group attached to the chiral centre 1) highest priority 2) next highest 3) next highest 202 4) lowest priority
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head2down higher atomic # (or atomic mass) gets the highest priority ( this is for the atom directly attached to the chiral carbon ) Question: Assign the priorities for all the groups in this molecule.
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